Add shadow memory mapping for Hexagon (32-bit architecture) and enable the TySan build for the Hexagon target. Hexagon uses a 4-byte shadow entry (PtrShift=2) with the shadow region at 0x80000000-0xBFFFFFFF (1GB). A 28-bit mask (kAppMemMsk) covers 256MB of app address space; addresses differing only in bits 28-31 alias in the shadow. kAppAddr is set to 0xC0000000 to size the mmap to exactly the 1GB shadow region.